Apparatus for and methods of measuring heat flux in a tunnel oven

ABSTRACT

Apparatus for and methods of meauring heat flux comprises first and second sensors arranged to pass through an oven in line astern. Each sensor measures the temperature difference across a thermally insulating layer by having one surface of each sensor being exposed to the heat flux, each sensor including thermocouple measuring junctions for measuring the temperature of the exposed surface. The first sensor is radiation-absorbing and the second sensor is reflecting. The apparatus includes additional thermocouple junctions for measuring the gas temperature and a device for recording data from the sensors and from additional thermocouple junctions. The signals from one of the sensors at one instant are correlated with the corresponding signals from the other sensor at a later instant, the time difference being the time taken for one sensor to reach a position formerly occupied by the other sensor.

The invention relates to the measurement of heat flux in ovens of thekind that are suitable for use in continuous processes in which materialto be heated (which may be in the form of discrete articles) istransported through the oven and is heated progressively during itspassage through the oven. Such ovens are known as tunnel ovens becausethey are elongate and have at one end an entrance through which thematerial is introduced into the oven and, at the other end, an exitthrough which the material is withdrawn. Tunnel ovens are used for avariety of purposes, for example, to dry material or to effect thebaking of food products.

The heat flux to be measured is that incident on a surface of thematerial to be heated. In general, the heat flux will have radiative andconvective components. The material will normally be supported frombelow on the upper run of an endless band conveyor and, where the bandis imperforate, the only exposed surface of the material will be itsupper surface. Where the band is a mesh, the lower surface of thematerial will be partly exposed to the heat flux, but it is the heatflux incident on the upper surface of the material that is here of primeconcern.

It is often important to measure separately the radiative and convectivecomponents of the heat flux, and that can be done by comparingmeasurements made using a radiation-absorbing surface sensor withmeasurements made using a reflective surface sensor. In each case, theheat flow can be determined by measuring, together with certain otherquantities, the temperature difference across a thermally insulatinglayer located between the exposed surface of the sensor and a heat sink.Essentially, the radiation-absorbing sensor responds to the total heatflux, whereas the reflective sensor responds only to the convectivecomponent of the heat flux.

Of course, neither sensor will behave either as a black body or as aperfect reflector; each sensor will both absorb and reflect radiationincident on it. It is strictly necessary only that the two sensorsshould have different absorptivities, but the more nearly theradiation-absorbing sensor behaves as a black body and the more nearlythe reflective sensor behaves as a perfect reflector the better theapparatus will perform. Throughout the specification, references to asensor being radiation-absorbing or radiation-reflecting are to beunderstood taking account of those facts.

An apparatus having two such sensors and arranged to operate in that wayis described in U.K. Patent Specification No. 2 183 346B. Typically, theheat flux will vary significantly along the length of a tunnel oven,with considerable variations occurring over relatively small distances.That will be especially marked when, for example, the heat flux isprimarily radiative and derives from burners or other heating elementsextending across the width of the oven at intervals along its length.Thus, the axial profile of the heat flux will show pronounced peaks andtroughs, and the apparatus of the invention is intended to enable theprecise form of that profile to be ascertained.

Because of the high spatial frequency of the fluctuations of the heatflux along the length of the oven, it seems clear that the two sensorsmust pass through the oven side-by-side. In a well designed oven,variations in the heat flux across the width of the oven will be small,but they will not usually be entirely negligible. Therefore, in order tominimise the effect of those variations across the width of the oven,the two sensors must be situated close together. It has now been found,however, that placing the two sensors sufficiently close together torender negligible the effects of transverse variations in the heat fluxresults in an inadequate degree of thermal insulation between them.

This invention provides apparatus for measuring heat flux in a tunneloven, which comprises first and second sensors and which can be conveyedthrough such an oven by the means used to convey through the oven thematerial to be heated with the first and second sensors in line astern,wherein each sensor comprises a layer of a thermally insulating materialand means for providing a signal representing the temperature differencebetween the two surfaces of the layer, one surface of the sensor beingin contact with a heat sink and the other surface of the sensor beingexposed, and each sensor including means for giving a signalrepresenting the temperature of the exposed surface of the sensor, thefirst sensor is radiation-absorbing and the second sensor is reflecting,the exposed surfaces of the two sensors are substantially coplanar andthe two sensors are spaced apart from one another, and which apparatusincludes means for giving a signal representing the gas temperature inthe vicinity of the exposed surfaces of the sensors, and which apparatusalso comprises means for periodically recording data derived from thesignals from each of the sensors and from the means for measuring thegas temperature in the vicinity of the two exposed surfaces, and whereinthe recording means is arranged to correlate the signals from one of thesensors at one instant, with the corresponding signals relating to theother sensor at a later instant, the difference in time between the twoinstants being, or being adjustable to be, equal to the time taken, whenin use the apparatus is being conveyed through the tunnel oven, for onesensor to reach a position formerly occupied by the other sensor.

In the apparatus of the invention, because the two sensors pass throughthe oven, not side-by-side, but in line astern, any effect thatvariations in the heat flux across the width of the oven may have on thedata gathered using the apparatus is not affected by the separationbetween the two sensors, with the result that a separation large enoughto secure adequate thermal isolation of the sensors can be used. Theminimum separation between the two sensors is advantageously at least 5mm and preferably at least 7 mm.

The manner in which the recording means operates avoids the need for aside-by-side arrangement of the sensors because, instead of recordingthe data from the two sensors at the same instant, when the two sensorsare at different positions along the length of the oven, the data fromthe two sensors are recorded at different instants so chosen that thedata from one sensor, when it is in one position, is compared with datafrom the other sensor when it is in the same position.

Advantageously, the means for measuring the gas temperature in thevicinity of the exposed surfaces of the two sensors comprises means formeasuring the gas temperature in the vicinity of the exposed surface ofone sensor and means for measuring the gas temperature in the vicinityof the exposed surface of the other sensor, and the recording means isarranged to correlate the signal representing the gas temperature in thevicinity of the exposed surface of each sensor with the other signalsfrom that sensor. Although the gas temperature will often not varygreatly over a distance equal to that separating the centres of the twosensors, the provision of separate gas temperature measuring means foreach sensor does give improved accuracy.

Advantageously, in each sensor, a thermopile constitutes the layer of athermally insulating material and the means for providing a signalrepresenting the temperature difference across the layer. A thermopilecomprises a piece of thin film of a plastic material in which areembedded thermocouples connected in series and so arranged that the coldjunction of each thermocouple is located close to one surface of thefilm and the hot junction of each thermocouple is located close to theother surface of the film. The use of a plurality of thermocouplesconnected in series both gives a spread of readings over the area wherethe thermocouple junctions are situated and, more importantly, gives alarger signal for a given temperature difference. Further, because ofthe construction of a thermopile, it is possible to arrange that it hasa small thermal capacity and hence a short response time.

Advantageously the exposed surfaces of the sensors are each rectangularin shape and they are so arranged that their major axes are parallel toeach other. The sensors are then spaced apart from each other in thedirection of the minor axes of their exposed surfaces so that, inoperation, the apparatus passes through the oven with the major axes ofthe exposed surfaces of the sensors perpendicular to the direction oftravel of the apparatus.

It is desirable that the area of the exposed surfaces of the sensorsshould be large because that permits the use of thermopiles with a largenumber of junctions and so provides a larger signal representing thetemperature difference across the layer of a thermally insulatingmaterial. On the other hand, it has been found that increasing thelinear dimension of the exposed surfaces of the sensors in the directionof travel of the apparatus through the oven impairs the spatialresolution of the apparatus for the variations in heat flux in thatdirection.

The length of the minor axis of the exposed surface of each sensoradvantageously does not exceed 75 mm and preferably does not exceed 50mm.

Different considerations apply to the linear dimensions of the exposedsurfaces of the sensors in a direction perpendicular to the direction oftravel of the apparatus through the oven, for it has been found thatincreasing those linear dimensions gives improved repeatability ofmeasurements taken during successive passages of the apparatus throughthe oven. That is believed to be because, as is observed above, therewill be some variations across the width of the oven in the magnitude ofthe heat flux, and so measurements made by the apparatus will in generaldepend on the precise position of the apparatus across the width of theoven. When the linear dimensions of the exposed surfaces of the sensorsin a direction perpendicular to the direction of travel of the apparatusthrough the oven are sufficiently large, the dependency of themeasurements on the position of the apparatus across the width of theoven is reduced so that repeatability can be achieved without the needfor precise positioning of the apparatus.

The length of the major axis of the exposed surface of each sensor isadvantageously at least 50 mm and preferably at least 75 mm.

The heat sink may comprise a metal block. Advantageously, the sensorsare mounted on portions of the metal block that are raised above theupper surface of the remainder of the block. Preferably, the raisedportions of the metal block are at least 5 mm above the upper surface ofthe remainder of the block.

Advantageously, an edge of the exposed surface of one of the sensors isclose to one edge of the upper surface of the apparatus. Then, byputting the apparatus in the oven so oriented that the two sensors passthrough the oven side-by-side, it can be arranged that the sensor thatis close to an edge of the upper surface of the apparatus is close to aside wall of the oven and, using the readings from that sensor only,information can be obtained about the heat flux close to that side wallof the oven. It is preferable that that sensor is the one having theradiation-absorbing upper surface.

The apparatus is advantageously provided with a removable extensionarranged to provide a horizontal surface substantially coplanar with theupper surface of the casing and extending away from the said one end ofthe casing. As is explained below, the provision of such a plateeliminates or reduces the effect on the measurements that mightotherwise result from the disturbance of the gas flow within the ovencaused by the presence of the apparatus. The plate is detached when, asdescribed above, the apparatus is to be used to measure the heat fluxclose to a side of the oven.

It is important that, when the apparatus is in use, the periodicallyrecorded data is recorded at sufficiently small time intervals.Advantageously, the recording means is arranged to log data from thesensors and from the means for measuring the gas temperature at afrequency of at least 2 sec⁻¹. Preferably, the said frequency is atleast 3 sec⁻¹.

Advantageously, the recording means is programmable to enable theapparatus to be used at different conveyor speeds. Preferably therecording means is arranged to be programmable by feeding into it datagiving the length of the oven with which the apparatus is to be used andthe time taken for the apparatus to be conveyed through the oven.

Advantageously, the recording means is a microcontroller comprising amicroprocessor, RAM and an EPROM.

The invention also provides a method of measuring the longitudinal heatflux profile in a tunnel oven, which comprises adjusting the delay inapparatus according to the invention in accordance with the speed atwhich material to be heated is conveyed through the oven, causing theapparatus to be conveyed through the oven and downloading the datarecorded by the apparatus to a computer.

One form of apparatus for measuring heat flux in a tunnel oven andconstructed in accordance with the invention will now be described, byway of example, with reference to the accompanying drawings, in which:

FIG. 1 is a plan view of the apparatus;

FIG. 2 is a plan view, on a larger scale than FIG. 1, of a part of theapparatus;

FIG. 3, is a diagrammatic cross-section taken on the line III—III inFIG. 2;

FIG. 4 is a view, on the same scale as FIG. 2, of a part of theapparatus seen from below with parts of the casing open or removed;

FIG. 5 is a block diagram; and

FIG. 6 is a side view of part of the apparatus with an extension fitted.

Referring to FIG. 1 of the accompanying drawings, the apparatuscomprises a casing, which is indicated generally by the referencenumeral 1 and has the shape of a shallow rectangular parallelepiped. Arectangular aperture 2 is formed in the upper wall 3 of the casing andclose to the leading end 4 of the casing, that is to say, the end of thecasing that, in use, enters the oven first. Secured to the sides of thecasing 1 are two carrying handles 5.

Within the aperture 3, there are located two sensors 6 a and 6 b,respectively. Each of the sensors 6 a and 6 b comprises a thermopile,that is to say, a plurality of thermocouples connected in seriesembedded in a lamina of a plastics material, with the measurementjunctions of the thermocouples exposed at one surface (here the uppersurface) of the plastics material and the reference junctions of thethermocouples exposed at the other surface (here the lower surface) ofthe plastics material. The two sensors 6 a and 6 b are each rectangular,and they are mounted side-by-side, but spaced apart from each other,with their longitudinal axes extending in the direction of the width ofthe casing 1.

The sensors 6 a and 6 b differ from one another in that the sensor 6 a,which is situated nearer to the leading end 4 of the casing 1 isradiation-absorbing, whereas the sensor 6 b, which is situated furtherfrom the leading end of the casing 1 is radiation-reflecting. The twosensors 6 a and 6 b are mounted on a block, which is indicated generallyby the reference numeral 7 (see FIG. 2). The block 7 is made of a metalwith a suitably high thermal conductivity, aluminium being preferred onthe grounds of cost and weight.

The sensors 6 a and 6 b are mounted on raised portions 7 a and 7 b,respectively, of the block 7 (see FIG. 3). The raised portions 7 a and 7b are rectangular as seen in plan and they are of the same width as thesensors 6 a and 6 b. The mounting of the sensors 6 a and 6 b on raisedportions of the block 7, and the fact that there is a significanthorizontal separation between the raised portions 7 a and 7 b providesgood thermal separation between the sensors. The block 7 is supportedon, and surrounded by, a mass of a thermally insulating material 8.

The linear dimensions of the exposed surfaces of the sensors 6 a and 6 bare approximately 40×100 mm, the upper surfaces of the raised portionsof the block 7 are approximately 6 mm above the upper surface of theremainder of the block, and the separation between the sensors isapproximately 8 mm.

From the thermopiles of each of the sensors 6 a and 6 b, a pair ofleads, 9 a and 9 b, respectively, pass through bores 10 a and 10 b,respectively, in the block 7, to the underside of the block. Mounted onthe upper surface of each of the sensors 6 a and 6 b is a thermocouplemeasurement junction 11 a or 11 b, respectively. A pair of leads 12 a or12 b runs from each of the thermocouple junctions 11 a and 11 b,respectively, to one of the bores 10 a and 10 b, respectively, andthence to the underside of the block 7.

Whereas the thermopiles give an e.m.f. indicative of the temperaturedifferences across, and hence of the heat flux through, the plasticslaminas of the sensors 6 a and 6 b, respectively, the thermocouplemeasurement junctions 11 a and 11 b enable a measurement to be made ofthe temperatures of the upper surfaces of the sensors 6 a and 6 b,respectively.

Each of the sensors 6 a and 6 b has associated with it a thermocouplejunction 13 a or 13 b, respectively, from which a flexible pair of leads14 a or 14 b runs, respectively, through a bore 15 a or 15 b,respectively, in the block 7 to the underside of the block 7. Thethermocouple measurement junctions 13 a and 13 b permit measurement ofthe gas temperature a short distance above the upper surface of theassociated sensors 6 a and 6 b, respectively. Because of the flexibilityof the pairs of leads 14 a and 14 b, the precise positions of thethermocouple junctions 13 a and 13 b, respectively, can readily beadjusted.

A part 16 of the underside of the casing 1 (see FIGS. 1 and 4) is hingedto the remainder of the casing at 17 and is releasably secured in itsclosed position by means of two pairs of flanges 18, each of which has(see FIG. 6) a threaded aperture for engagement by a closure screw 19when, in the closed position of the part 15, the flanges of each pairare adjacent to each other. As seen in FIG. 4, the part 16 is in itsopen position. Further, in order to give access to the underside of theblock and its immediate surroundings, a removable panel (which is heldin position by screws) is provided in the underside of the case 1. Theremovable panel is not shown and FIG. 4 shows the underside of thecasing with the panel removed.

The underside of the block 7 is provided with channels 20 and 21,respectively. The channel 20 runs from the lower ends of the bores 10 aand 10 b to the rear edge of the underside of the block 7, and affords apassage for the pairs of leads 9 a and 9 b from the series-connectedthermocouples of the thermopiles of the two sensors 6 a and 6 b,respectively, and for the pairs of leads 12 a and 12 b from thethermocouple measurement junctions 11 a and 11 b mounted on the uppersurface of the sensors 6 a and 6 b. The channel 21 runs from the lowerends of the bores 15 a and 15 b to the rear edge of the underside of theblock and affords a passage for the pairs of leads 14 a and 14 b fromthe thermocouple measurement junctions 13 a and 13 b that are providedto measure the gas temperatures above the upper surfaces of the sensors6 a and 6 b, respectively.

After leaving the channel 20, the four pairs of leads 9 a, 9 b, 12 a and12 b pass through a continuation of that channel formed in theinsulating material 8 and then through the interior of a helical spring22 which, while being flexible, serves to protect them mechanically.After leaving the interior of the helical spring 22, the eightindividual leads making up the four pairs of leads 9 a, 9 b, 12 a and 12b are connected to conductors on a PCB, that is to say, a printedcircuit board (not shown) via eight of the twelve terminals of a screwclamp terminal 23.

The body of the screw clamp terminal 23 is made of a plastics materialthat combines relatively high thermal conductivity with sufficientlyhigh electrical resistivity to ensure substantially complete electricalinsulation between the individual terminals. The screw clamp terminal 23is surrounded on three sides by an aluminium heat sink 24, which ensuresa high degree of temperature uniformity as between the individualterminals. Further, a thermistor is provided at 25 to give a signalindicative of the temperature of the aluminium heat sink 24.

After leaving the channel 21, the two pairs of leads 14 a and 14 b passthrough a continuation of that channel formed in the insulating materialto a cavity 26, where the two individual leads making up the pair ofleads 14 a are connected to a 2-pin plug 26 a, and the individual leadsof the pair of leads 14 b are connected to another 2-pin plug 26 b. Theplugs 26 a and 26 b form part of a plug-and-socket connector. From thesocket part of the connector (which part is not shown), four individualleads go to the remaining four terminals of the screw clamp terminal 23.

The metals of which the electrically conducting parts of the plugs 26 aand 26 b are made are the same as those of which the individual leads ofthe pairs of leads 14 a and 14 b are made, and the same is true of theelectrically conducting parts of the associated sockets (not shown) andof the leads from the sockets to the screw clamp terminal 23. Further,the arrangement is such that the use of the plug-and-socket connectorsdoes not result in the introduction of further, unwanted thermocouplejunctions. Then, so far as thermoelectric effects are concerned, theleads from the sockets to the screw clamp terminal 23 can be regarded asmere extensions of the individual leads of the pairs of leads 14 a and14 b.

Because the thermocouple measurement junctions 13 a and 13 b, which areused to measure the gas temperature, are situated above the uppersurfaces of the sensors 6 a and 6 b, they are prone to damage. The useof the plugs 26 a and 26 b, together with the associated sockets (notshown) facilitate replacement of the leads 14 a and 14 b and hence ofthermocouple measurement junctions 13 a and 13 b as and when required.

In the case of the pairs of leads 12 a and 12 b from the thermocouplemeasurement junctions 11 a and 11 b, respectively, the junctions betweenthe individual leads of those pairs of leads and the individualterminals of the screw clamp terminal 23 constitute the referencejunctions of the thermocouples. In the case of the thermocouples used tomeasure the gas temperature, which include the thermocouple measurementjunctions 13 a and 13 b, the reference junctions are those between theleads from the plug-and-socket connector and the individual terminals ofthe screw clamp terminal to which they are connected.

The printed circuit board and its associated circuitry (both not shown)constitute the apparatus for measuring the e.m.f.s generated by thethermocouples (including those in the thermopiles), and the junctionsbetween the conductors on the printed circuit board constituteadditional junctions associated with the measuring apparatus and, asrequired, the arrangement ensures that those junctions are maintained atsubstantially the same temperature.

In the case of the pairs of leads 9 a and 9 b from the thermopiles ofthe sensors 6 a and 6 b, both the junctions between the individual leadsof those pairs of leads and the individual terminals of the screw clampterminal 23, and the connections between the screw clamp terminal andthe printed circuit board, constitute additional junctions associatedwith the measuring apparatus. Again, the additional junctions aremaintained at substantially the same temperature, as is required.

The apparatus is battery-operated, the batteries (which are not shown)being housed within the interior of the casing 1.

The signals that are carried by conductors on the printed circuit boardfrom the individual terminals of the screw clamp terminal 23, andsignals from the thermistor 25, pass to an analogue-to-digital converter27, as is indicated schematically in FIG. 5. The corresponding digitalsignals emanating from the analogue-to-digital converter 27 are passedto a recording means in the form of a microcontroller 28 (see FIG. 5),which includes a microprocessor, an EPROM and RAM.

The program, together with basic numerical data, are stored in theEPROM. The RAM is used by the program, and it also serves to store thedata acquired by the apparatus during a passage of the apparatus throughthe oven. After the apparatus has passed through the oven, the data thathas been acquired during the passage and stored in the RAM is downloadedto a computer.

The data is recorded periodically at intervals of 0.25 sec, that is tosay, at a frequency of 4 sec⁻¹. If that frequency is not high enough, itwill be found that the results obtained are not repeatable.

The microcontroller 28 so records the data from the two sensors 6 a and6 b, and from the associated thermocouples (those having the measurementjunctions 13 a and 13 b) for measuring the gas temperature in the regionof the sensors, that the readings from the leading sensor 6 a at a giveninstant are correlated with the readings from the following sensor 6 bat a later instant. The delay between the two instants is equal to thetime taken for the apparatus to travel, in the oven, a distance equal tothe separation between the centres of the two sensors.

In order to enable the apparatus to be used in ovens in which thearticles to be heated are conveyed at different speeds, or in a givenoven in which that speed is adjustable, the microcontroller 28 can beset so that the delay is appropriate for the speed. Because the lengthof the oven and the time taken for an article to be conveyed through theoven are easily measurable quantities, the microcontroller 28 is suchthat the appropriate delay can be set simply by entering that length andthat time.

In ovens in which the heating has a significant convective component,the gas within the oven will have a significant velocity, and theconveying of the apparatus through the oven will disturb the gas flowover the apparatus, especially in the vicinity of the leading end 3 ofthe casing 1, and therefore in the vicinity of the sensors 6 a and 6 b.Such a disturbance of the gas flow over the sensors 6 a and 6 b mayresult in there being a difference between the magnitudes of theconvective component of the heat flux incident on the two sensors, eventhough the comparison between those magnitudes is made, not at the sametime, but at the same position of the two sensors. If that occurs, therewill be an error in the measurement of the radiative and convectivecomponents of the heat flux, and probably also an error in themeasurement of the total heat flux.

In order to prevent or reduce the risk of errors resulting from adisturbance of the gas flow pattern by the apparatus as it passesthrough the oven, the apparatus is provided with a detachable extension,which is indicated generally by the reference numeral 29 (see FIG. 6).The extension 29 consists of a rectangular plate 30 and at its rear enda downwardly extending flange 31 formed with apertures to receive bolts32 for releasably securing the extension to the front of the casing 1.

When the extension 29 is secured to the casing 1, the upper surface ofthe rectangular plate 30, which extends over the entire width of thecasing 1, is flush with the upper surface of the casing. As has beenpointed out above, the main disturbance of the gas flow caused by thepassage of the apparatus through the oven is in the vicinity of theleading end of the apparatus which, when the extension 29 is not fitted,is in the vicinity of the sensors 6 a and 6 b. When the extension 29 isfitted, the leading end of the apparatus is the leading edge of theplate 30, so that the region of significant disturbance of the gas flowis displaced forwards, away from the sensors 6 a and 6 b, by a distanceequal to the length of the plate.

In use, the apparatus is placed on the conveyor that is used to conveymaterial to be heated through the oven, with the sides of the casing 1extending in the direction of movement and the leading end 4 in front.The extension 29 is normally fitted. The microcontroller 28 has beenprogrammed to give the appropriate delay as explained above. Also,before the apparatus is allowed to enter the oven, the upper surfaces ofthe sensors 6 a and 6 b must be brought up to a temperature exceedingthe dewpoint of the gas in the oven. That can be achieved by exposingthose surfaces, but not the remainder of the apparatus, to atemperature-controlled air-flow. If the upper surfaces of the sensors 6a and 6 b are not brought up to the required temperature, there will becondensation on the upper surfaces of the sensors, and the upper surfaceof the sensor 6 b will become significantly less reflective, so that thereadings will be seriously distorted.

When the apparatus has passed through the oven, the data logged by themicrocontroller 28 is downloaded to a computer, and the heat fluxprofile along the length of the oven, for both the convective componentand the radiative component of the heat flux, is obtained.

If, as is sometimes the case, it is desired to investigate the heat fluxprofile close to a side of the oven, the extension 29 is removed byreleasing the bolts 32, and the apparatus is so placed on the conveyorthat the sides of the casing 1 extend across the width of the oven andthe end 3 of the casing is close to the side of the oven. Themicrocontroller 28 is then programmed to record signals from the sensor6 a only, instead of recording signals from both of the sensors 6 a and6 b.

What is claimed is:
 1. Apparatus for measuring heat flux in a tunneloven comprising conveying means to convey through the oven material tobe heated and containing a gas which surrounds the material to beheated, the apparatus comprising first and second sensors and which areconveyed through such an oven by the conveying means with the first andsecond sensors in line astern, wherein each sensor has a first surfaceand a second surface and comprises a layer of a thermally insulatingmaterial between said first and second surfaces and means for providinga signal representing a temperature difference across the layer, thefirst surface of each sensor being in contact with a heat sink and thesecond surface of each sensor being exposed, and each sensor includingmeans for giving a signal representing the temperature of the exposedsurface of the sensor, the first sensor is radiation-absorbing and thesecond sensor is reflecting, the exposed surfaces of the two sensors aresubstantially coplanar and the two sensors are spaced apart from oneanother, and which apparatus includes means for giving a signalrepresenting the gas temperature in the vicinity of the exposed surfacesof the sensors, and which apparatus also comprises means forperiodically recording data derived from the signals from each of thesensors and from the means for measuring the gas temperature in thevicinity of the two exposed surfaces, and wherein the recording means isarranged to correlate the signals from one of the sensors at oneinstant, with the corresponding signals relating to the other sensor ata later instant, the difference in time between the two instants being,or being adjustable to be, equal to the time taken, when in use theapparatus is being conveyed through the tunnel oven, for one sensor toreach a position formerly occupied by the other sensor.
 2. Apparatus asclaimed in claim 1, wherein the minimum separation between the twosensors is at least 5 mm.
 3. Apparatus as claimed in claim 2, whereinthe minimum separation between the two sensors is at least 7 mm. 4.Apparatus as claimed in claim 1, wherein the means for measuring the gastemperature in the vicinity of the exposed surfaces of the two sensorscomprises means for measuring the gas temperature in the vicinity of theexposed surface of one sensor and means for measuring the gastemperature in the vicinity of the exposed surface of the other sensor,and wherein the recording means is arranged to correlate the signalrepresenting the gas temperature in the vicinity of the exposed surfaceof each sensor with the other signals from that sensor.
 5. Apparatus asclaimed in claim 1, wherein, in each sensor, a thermopile constitutesthe layer of a thermally insulating material and the means for providinga signal representing the temperature difference across the layer. 6.Apparatus as claimed in claim 5, wherein the exposed surfaces of thesensors are each rectangular in shape and they are so arranged thattheir major axes are parallel to each other.
 7. Apparatus as claimed inclaim 6, wherein the length of the minor axis of the exposed surface ofeach sensor does not exceed 75 mm.
 8. Apparatus as claimed in claim 7,wherein the length of the minor axis of the exposed surface of eachsensor does not exceed 50 mm.
 9. Apparatus as claimed in claim 6,wherein the length of the major axis of the exposed surface of eachsensor is at least 50 mm.
 10. Apparatus as claimed in claim 9, whereinthe length of the major axis of the exposed surface of each sensor is atleast 75 mm.
 11. Apparatus as claimed in claim 1, wherein the heat sinkcomprises a metal block.
 12. Apparatus as claimed in claim 11, whereinthe sensors are mounted on portions of the metal block that are raisedabove the upper surface of the remainder of the block.
 13. Apparatus asclaimed in claim 12, wherein the raised portions of the metal block areat least 5 mm above the upper surface of the remainder of the block. 14.Apparatus as claimed in claim 1, having walls which define the lateralboundaries of the apparatus and an upper surface, the walls and theupper surface meeting to form edges, wherein an edge of the exposedsurface of one of the sensors is close to an edge formed by the meetingof the upper surface of the apparatus with the walls.
 15. Apparatus asclaimed in claim 14, wherein the sensor of which an edge of the exposedsurface is close to an edge of the upper surface of the apparatus is thesensor of which the exposed surface is radiation-absorbing. 16.Apparatus as claimed in claim 14, wherein the apparatus is provided witha removable extension arranged to provide a horizontal surfacesubstantially coplanar with the upper surface of the casing andextending away from the said one end of the casing.
 17. Apparatus asclaimed in claim 1, wherein the recording means is arranged to log datafrom the sensors and from the means for measuring the gas temperature ata frequency of at least 2 sec⁻¹.
 18. Apparatus as claimed in claim 17,wherein the said frequency is at least 3 sec⁻¹.
 19. Apparatus as claimedin claim 1, wherein the recording means is programmable to enable theapparatus to be used at different conveyor speeds.
 20. Apparatus asclaimed in claim 19, wherein the recording means is programmable byfeeding into it data giving the length of the oven with which theapparatus is to be used and the time taken for the apparatus to beconveyed through the oven.
 21. Apparatus as claimed in claim 1, whereinthe recording means is a microcontroller comprising a microprocessor,RAM and an EPROM.
 22. A method of measuring the longitudinal heat fluxprofile in a tunnel oven, which comprises the step of providing anapparatus for measuring heat flux in a tunnel oven comprising conveyingmeans to convey through the oven material to be heated and containing agas which surrounds the material to be heated, the apparatus comprisingfirst and second sensors and which can be conveyed through such an ovenby the conveying means with the first and second sensors in line astern,wherein each sensor has a first surface and a second surface andcomprises a layer of a thermally insulating material between said firstand second surfaces and means for providing a signal representing atemperature difference across the layer, the first surface of eachsensor being in contact with a heat sink and the second surface of eachsensor being exposed, and each sensor including means for giving asignal representing the temperature of the exposed surface of thesensor, the first sensor is radiation-absorbing and the second sensor isreflecting, the exposed surfaces of the two sensors are substantiallycoplanar and the two sensors are spaced apart from one another, andwhich apparatus includes means for giving a signal representing the gastemperature in the vicinity of the exposed surfaces of the sensors, andwhich apparatus also comprises means for periodically recording dataderived from the signals from each of the sensors and from the means formeasuring the gas temperature in the vicinity of the two exposedsurfaces, and wherein the recording means is arranged to correlate thesignals from one of the sensors at one instant with the correspondingsignals relating to the other sensor at a later instant, the differencein time between the two instants being, or being adjustable to be, equalto the time taken, when in use the apparatus is being conveyed throughthe tunnel oven, for one sensor to reach a position formerly occupied bythe other sensor and the step of adjusting the difference in timebetween the two instants in accordance with the speed at which materialto be heated is conveyed through the oven, causing the apparatus to beconveyed through the oven and downloading the data recorded by theapparatus to a computer.
 23. A method of measuring the longitudinal heatflux profile in a tunnel oven, the tunnel of the oven containing a gas,the method comprising the steps of: (a) conveying through the tunneloven a heat flux measuring apparatus comprising a heat flux sensorhaving a radiation absorbing surface and a heat flux sensor having aradiation reflecting surface, each sensor comprising a layer ofthermally insulating material and means for providing a signalrepresenting a temperature difference across the layer, the apparatusfurther comprising means for measuring the gas temperature in thevicinity of the sensors and being conveyed through the tunnel oven withthe sensors in line astern; b) obtaining a signal from each of thesensors representing the temperature difference across its insulatinglayer; c) recording data derived from the signals from the sensors andfrom the means for measuring the gas temperature; d) determining a timedelay taken for one sensor to reach a position formerly occupied by theother sensor; and e) correlating the signal from one of the sensors atone instant with the signal from the other sensor at another instant atthe end of a time period equal to the time delay.